So-called activated sludge treatment processes which comprise introducing waste water into the reaction tank in which activated sludge is present followed by aeration are widely used as a purification treatment process for waste water containing organic matter.
In the operation of the activated sludge treatment process, however, an excess sludge is formed inevitably in the reaction tank, necessitating treatment of the excess sludge by a use of the dehydrator. Because of large amounts of the excess sludge formed in the standard reaction tank, this process sometimes involves additionally the digestion treatment of the excess sludge under anaerobic and/or aerobic condition in a digestion tank. In this respect, refer to Journal of Sewage Association, 22 (248), 42 (1985), for example.
Alternatively, the amount of excess sludge formed is lessened in some cases by employing the so-called long time aeration process wherein the capacity of the reaction tank is sufficiently increased to reduce the sludge load.
In practicing the dehydration of sludge by the use of the dehydrator, however, the cost of initial equipments to be provided therefor is great and much skill is required in many cases for operating the dehydrator and, moreover, there is such a problem that for the disposal of the dehydrated sludge, there is no other alternative but to incinerate the dehydrated sludge to be disposed of or use the same for land reclamation.
In practicing the digestion of excess sludge, on one hand, an aerobic digestion process employed therefor, which process effects the digestion of excess sludge under conditions where oxygen is sufficiently present, involves such a problem that the digestibility attained thereby is as low as about 2-4% per day and accordingly a large sized digestion tank must be used if the amount of excess sludge formed is large. Furthermore, in an anaerobic digestion process wherein the digestion of the formed excess sludge is carried out under conditions where oxygen is deficient, the digestibility attained is as high as about 10% per day. This process, however, involves such a problem that because undigested sludge aggregates and decreases in dehydration properties, disposal of the undigested sludge to be discarded becomes difficult, or a bad smell is emitted, or simplification of the equipment to be employed is difficult. In either of the above-mentioned digestion processes, moreover, there is involved such a serious problem that the rate of digestion markedly decreases at the time when 45-50% of the excess sludge has been digested, and when the digested excess sludge exceeds 55%, the self-digestion coefficient decreases, resulting in practical nonfulfillment of digestive function.
For these reasons, under the present circumstances involving the above-mentioned prior art processes, even when the excess sludge is digested by the use of the digestion tank, about half of the amount of excess sludge formed in the reaction tank remains undigested.
Where the log time aeration process is intended to treat the excess sludge, on one hand, there is such a problem that a large-sized reaction tank must be used and consequently the space allotted to said reaction tank naturally becomes large and, moreover, a large energy consumption is needed for maintaining large amounts of aeration required for the purpose intended.